Unforunately this is not really uncommon among lifters, especially on the big lifts like deads/squats. I personally get these myself when getting near my max on deadlifts and I know what your talking about, its painful and can scare the crap out of you. My advice is to simply listen to your body and if you feel one coming on to stop the lift and play it safe, you don't want to be trapped under the weight like you mentioned. Paying attention to my breathing has also helped me these.

Here is some general info that may help:

Benign Exertional Headache (Weight lifter's cephalgia)
Weight lifters headache is generally sudden in onset and occurs during active lifting. In many cases, the weight training exercise being performed at the time of headache onset was the bench press or leg press. It usually occur as a result of the maximal exertion during weight lifting (Powell 1982). The pain is described as burning or boring in quality and localized to the posterior head and neck however can radiate to the parietal and temporal parts of the occiput. Though onset is abrupt, the headache may persist for several hours or even several days gradually resolving. No clear cause has been identified in medical studies; the presumed mechanism is stretching of the cervical ligaments and tendons with development of excessive muscle contraction or vascular (due to high blood pressure). It gradually declines, leaving a residual ache that usually lasts for 4-6hours. In subsequent weeks to months, the headache recurs with exertion. The patient has no history of migraine and a normal neurologic exam.

Its important that you go see your doc. He should give u a physical focussing particularly on a neurological exam to asses of any signs of focal lesions and rule out serious causes of headaches. Do you have hypertension (high blood pressure)? Either way, your blood pressure will most definitely be measured.

Your breathing, or lack of it, could cause an massive elevation in your blood pressure causing vascular headaches (seen in weight lifters whose heart rate doubles in less than 10 seconds when performed extremely intense activity - Studies of weight lifters demonstrate that, with maximal lifts, systolic blood pressure may reach levels above 400 mm Hg and diastolic pressures above 300 mm Hg.).

What can you do about it?
Try diff exercises for those bodyparts.
Or Same exercises but lighter weights.
Pay attention to breathing.
Sometimes a NSAID painkiller is helpful.

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Here is a study done a while back:

Ann Emerg Med. 1982 Aug;11(8):449-51.

Weight lifter's cephalgia.

Physical exertion can cause migraine or the benign exertional headache. The case is presented of a 30-year-old male weight lifter who experienced excruciating headache while doing leg presses on a weight machine. Physical examination was normal, as were subsequent examinations. The patient resumed training with lighter weights and did not again experience headaches. Some experts estimate that up to 4% of the adult population engages in weight lifting occasionally, and thus this malady may be more common than is realized.